How Heat Pump Technology Works
The best analogy for the way a heat pump works is to consider what happens when you pump up a bicycle tyre. When you pump the air into the tyre, the end of the pump becomes hot. This is a general rule of physics. If you increase the pressure of a gas, it becomes hotter as the molecules in the gas are forced closer together. The reverse is also true; the lower the pressure the cooler a gas becomes. This is known as an adiabatic process and is the scientific principal behind the workings of a heat pump.
The main component of a heat pump is a compressor which pressurises a refrigerant gas in a closed circuit, causing it to become hot. The hot gas is piped to a heat exchanger where it heats up the water. As the gas gives up its heat to the water, it reverts back to a liquid state. This liquid, which is still under pressure, passes through a tiny nozzle into a second heat exchanger or evaporator. The evaporator connects to the compressor input, completing the circuit.
The low pressure in the evaporator causes the liquid refrigerant that is being forced through the expansion valve, to fall in temperature to between 2°C and 6°C. This cold refrigerant is then warmed by the ground water from the borehole (or anti-freeze in a closed loop system) which is at around 8°C to 12°C. Because the evaporator is connected to the suction side of the compressor, it is at a low pressure and the refrigerant turns back into a gas. It is then re-pressurised by the compressor and the cycle begins again. The borehole water (or antifreeze) is cooled by around 6°C by this process and is returned to the ground where it is reheated.
The efficiency of heat pumps are quantified by their co-efficiency of performance or COP. This is how much additional heat energy is provided by the ground compared to how much input energy is required to run the heat pump.
Example:
Heat pump kilowatt ratings are the amount of energy they are capable of delivering. If we assume we have a 12 kW rated heat pump which has a COP of 4, it will need 3 kW of energy to run it and 9 kW of energy will be extracted from the ground to deliver the 12 kW of heating.
